The present invention relates to a metal gasket, what is called a cylinder head gasket, for use in sealing the joint surfaces of the cylinder head and the cylinder block of an internal combustion engine.
There is well known a cylinder head gasket constructed by laminating metal plates having combustion chamber hole(s), bolt hole(s), hole(s) for cooling water passage and oil passage and the like, wherein the upper plate 101, intermediate plate 104, thickness-adjusting plate 103 and lower plate 102 are laminated, for example, as shown in FIG. 1, and wherein the thickness of the peripheral portion of the combustion chamber hole 105 of the gasket is made larger than that of the remaining portion of the gasket by providing a bent portion 106 which is formed by turning back the upper plate 101, and by making the thickness of the upper plate 101 larger than that of the thickness adjusting plate 103. When such a gasket is attached to an engine and fastened, a high annular sealing pressure zone takes place around the peripheral portion of the combustion chamber hole. It is also well known that, in the above gasket, a bead portion for surrounding the combustion chamber hole is formed, for example, in the intermediate plate 104 so as to add a high pressure resulting from the resilience of the bead portion to the high pressure resulting from the above difference in thickness of the gasket so that the sealing effect of the gasket can be increased. In this regard, the thickness of a certain portion of a gasket referred to in the present specification means the sum of the respective thickness of metal plates laminated and that of a rigid resin layer.
In the metal gasket as constructed above, the difference in thickness between the peripheral portion of the combustion chamber hole and the remaining portion of the gasket is determined by difference in thickness between the metal plates 101 and 103, and therefore, it is difficult to construct the gasket with a difference in thickness between both the above portions of the gasket so that a desired pressure distribution can be created.
A fastening force the gasket receive when fastened differs partially depending on a position, that is, on whether a portion of the gasket locates in the vicinity of a bolt hole or is spaced away from the bolt hole. Accordingly, the sealing pressure to the peripheral portion of the combustion chamber hole differs partially between both positions as above. In order to uniform a sealing pressure onto the peripheral portion of the combustion chamber hole, it is required to vary the thickness of the peripheral portion of the combustion chamber hole of the gasket in accordance with a distance from a bolt hole. Such a thickness adjustment is, however, practically impossible in the above metal gasket.
The assignee of the present invention previously proposed the following metal gasket in Japanese Patent application No. 8-138196 (Japanese Laid-Open Patent Publication No. 9-317890 issued on Dec. 12, 1997). That is, the metal gasket comprises, as shown in FIG. 2, a metal plate 100; a groove 107 formed in one surface thereof surrounding the combustion chamber hole 105; a rigid resin layer 108 of, for example, epoxy resin, so formed in the groove 107 as to be raised from the surface of the metal plate 100; and a bead portion 109 for surrounding the combustion chamber hole 105, said bead portion 109 being formed outside the rigid resin layer 108 with its height greater than that of the rigid resin layer 108. The assignee also proposed that the height of the rigid resin layer 108 from the surface of the metal plate is varied in accordance with a position on the peripheral portion of the combustion chamber hole 105: the height of the rigid resin layer 108 is lower in the vicinity of a bolt hole and it is higher between each of bolt holes. The rigid resin layer 108 serves to increase the thickness of the gasket at the peripheral portion of the combustion chamber hole so that a higher annular sealing pressure zone can take place at the peripheral portion of the combustion chamber hole. The rigid resin layer can be formed with a desired thickness, and the thickness of the rigid resin layer can be varied easily in accordance with a position on the peripheral portion of the combustion chamber hole.
However, the metal gasket having such a rigid resin layer has a problem that, since the rigid resin layer is exposed directly to a high temperature combustion gas during operation of an engine, there is a fear that the rigid resin layer will deteriorate and burn during a long term of use. Also, the metal gasket has another problem that, where the rigid resin layer comes into contact with the joint surfaces of a cylinder head and a cylinder block when the gasket is attached to an engine, the rigid resin layer is worn down by friction with the joint surfaces due to vibrations of the cylinder head and the cylinder block during operation of the engine and thermal expansion or contraction of the cylinder head and the cylinder block accompanying the start or stop of operation of the engine. Thus, the gasket loses its function and decreases in sealing performance during a long term of use. Further, the deterioration proceeds more due to interaction between attack of a combustion gas and the friction with the joint surfaces.